This invention relates generally to a method improving storage capacity for an adsorbable gas and, more particularly, to effecting such storage in a composite adsorption medium wherein a bed of solid adsorbent particles has already adsorbed therein a more strongly adsorbent gas.
Adsorptive storage of gases is more efficient than compression storage due to the reduced pressure levels. At low pressures, the container cylinders need not be heavy-wall and the compressor can be a single-stage rather than multi-stage compressor reducing the fixed costs. In addition, operating costs of the compressors are greatly reduced because of the lower discharge pressures employed to desorb the adsorbed gas. Adsorptive storage can be utilized for any gas which has a critical temperature higher than ambient temperature, which, therefore, cannot be liquefied without refrigeration. At present there is only commercial application for adsorptive storage of natural gas. The desired storage can be effected by contacting the gas stream with a bed of solid adsorbent particles employing fixed or static bed operation as well as moving bed operation. Of major consideration in developing and optimizing adsorptive storage systems is the storage efficiency of the gas. Volumetric storage capacity is a recognized criteria for storage efficiency which can be defined as the number of bed volumes discharged at atmospheric conditions per storage bed volume. As such, the volume storage capacity represents a ratio of two volumes, for example, cubic feet per cubic feet.
A low-pressure system for storing gaseous hydrocarbon fuel in automotive vehicles is disclosed in U.S. Pat. No. 4,523,548. Multiple cylinders are employed containing either adsorbent or absorbent material including molecular sieves, activated carbon, zeolite compounds, various clays and silica gel which store the hydrocarbon gas at pressures ranging from approximately 100 psig to approximately 400 psig. The gas is filtered before storage with sorptive filtering means to remove undesirable constituents such as odorant additives (dimethyl sulfide and the like) which are frequently included in natural gas as a safety measure. The reported purpose (column 14, lines 19-22) for removing such constituents is further said to be maximizing the capability of the storage vessels to sorptively store the lighter hydrocarbons such as methane. A different storage system for gaseous hydrocarbon fuels is disclosed in U.S. Pat. No. 4,817,684. An illustrated gas mixture of methane containing an additive such as dimethyl sulfide, tetrahydrothiothene (THT), tertiary butyl mercaptan (TBM) and others is adsorbed at elevated pressures within a storage vessel containing a bed of solid adsorbent particles which has been preconditioned at a lower pressure with the additive. Storage of the gas mixture within the storage vessel is said to proceed in accordance with isothermic curves depicted in FIG. 2 of said prior art reference (column 5, lines 26-32).
In all such type adsorptive storage systems, a considerable amount of the adsorbable gas can also be left on the solid adsorption medium (contained in the cylinder or other type storage vessel) after the depletion pressure is reached. As high as fifty percent of the adsorbed gas contained in the storage vessel at charge can be retained in the adsorption bed at depletion depending on the particular adsorbable gas and solid adsorption medium being employed. The adsorbed gas left after depletion is not usable in any normal storage application since it will require vacuum application to remove it. Accordingly, there still remains a serious need for better means to desorb more of the stored gas at atmospheric conditions of pressure and temperature.
It is therefore an object of the present invention to provide a more effective method of storing an adsorbable gas in a solid adsorption medium so that less gas is retained upon removal therefrom.
It is another object of the present invention to provide a method which increases the storage capacity of an adsorptive storage system by removing more of the adsorbed gas.
It is still a further object of the present invention to provide a method of storing an adsorbable gas at elevated pressures in a bed of solid adsorbent particles so that more of the adsorbed gas is released when pressure is reduced.
These and other objects as well as advantages of the present invention will become more apparent from the following detailed description being provided upon the preferred embodiments.